The present invention relates generally to the use of cosmic radiation as a measurement tool, and more particularly, the ability to use the attenuation of cosmic gamma radiation passing through a given accumulation of snow to determine its water equivalent.
In geographic areas where the water supply is derived mainly from snow runoff, the ability to forecast accurately the snowpack water equivalent is a critical factor in the prudent allocation of water resources. Consequently, there has been a long felt need for an inexpensive, reliable and practical method for determining these data. In the United States, the annual snowpack in the Sierra Nevada, Cascade and Rocky Mountain ranges provide the bulk of the water supply for most of the western states. Also, significant parts of New England are serviced by snow runoff from the Appalachians. For these same reasons, many foreign countries are also interested in snow hydrology, including Canada, several European countries, Russia, and Japan.
In a geographic area such as California, on the slopes of the Sierra Nevada alone, there are currently four hundred snow survey sites that are monitored for snow water content to assist in annual water allocation forecasts. One hundred twenty of the sites are automated. These sites use snow pillows (a grid of four 3 foot square hydraulically suspended steel pads that weigh the snow deposited on them) connected to a weather station. The snow pillows are physically imposing and incur high installation as well as high annual maintenance costs; their size makes them very difficult to install in remote locations. The remainder of the snow survey sites require "core samples" that are taken at 2 week intervals during the snow season. As the demand on water resources increases there is a corresponding need to automate more snow survey sites in order to forecast more accurately snow runoff and allocate resources appropriately; in particular, to have more sampling sites, to automate data collection and to have the capability of recovering data as it is being generated. However, to automate these sites with current technology (e.g. snowpillows) would be expensive because of the physical size of the equipment, along with the difficulty in placing such equipment in remote locations. Because of the mountainous terrain generally encountered in establishing or maintaining current automated snow survey sites, support personnel and equipment must often be transported to remote sites by helicopter, an expensive and often hazardous activity. In spite of these dificulties, the need for these data is urgent enough to justify the expense of installation and maintenance. It is estimated that there are approximately 1300 automated snowpack sites in the United States having a total annual estimated maintenance cost in excess of one million dollars. The California State Department of Water Resources alone spends in excess of one hundred thousand dollars annually in maintenance costs for its 120 snow pillows.
In addition to the techniques described above, radioactive techniques for measuring the snow water equivalent have been reported in the literature. Smith et al. in Canadian Patent No. 861635 describes a technique that uses active sources of gamma radiation which are biologically dangerous and pose obvious environmental and safety concerns. A second technique has been described by Wada et al. in U.S. Pat. No. 4,047,042 which utilizes ground based detectors for monitoring cosmic neutron radiation. This technique suffers from accuracy limitations due to very low counting rates and interference due to soil moisture. A third technique, described by Abelentsev et al. in U.S. Pat. No. 4,992,667, utilizes fixed gamma ray detectors located above the snowpack to monitor the variation in the natural emissions of gamma radiation from the earth (as opposed to secondary background cosmic gamma radiation) versus snow depth. Terrestrial gamma rays are low energy radiation, typically having energies less than 200 keV. As a consequence, they are rapidly attenuated by water. One measure of the attenuation of gamma rays is the e-folding depth, i.e., the depth at which the gamma ray intensity falls to 1/e. In water, terrestrial gamma rays have an e-folding depth of approximately 9 cm and thus cannot be used to monitor snowpack with a water equivalent of more than 30-40 cm. Consequently, this technique is limited to snow depths less than is typical in most mountain areas. Furthermore, because of the significant atmospheric attenuation of terrestrial gamma radiation monitoring by aircraft must be carried out at low altitudes. It cannot be used in deep mountainous terrain because of safety factors. For reasons discussed above, deep mountainous snowpacks also drastically attenuate the terrestrial gamma radiation, leading to often unreliable and inaccurate data. Because of the urgent need for these data and despite the drawbacks set forth above, water equivalent data generated from measurements of terrestrial gamma ray attenuation are used to calculate snowmelt and update forecasting models. The present invention discloses method and apparatus for determining the water equivalent of snowpack which, by measuring the attenuation of secondary cosmic gamma radiation, has significant advantages over existing techniques.